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Geological control on the morphology of estuarine shore platforms: Middle Harbour, Sydney, Australia
Geomorphology 114 (2010) 71–77
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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h
Geological control on the morphology of estuarine shore platforms: Middle Harbour, Sydney, Australia D.M. Kennedy ⁎ School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand
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Article history: Accepted 5 February 2009 Available online 15 February 2009 Keywords: Shore platform Estuary Erosion Joint Sandstone Rock coast Inheritance
a b s t r a c t Middle Harbour, Sydney, Australia is a microtidal environment formed in resistant Triassic quartzose sandstone (mean N-type Schmidt Hammer rebound value of 37 ± 6). Wave energy is dominated by deepwater ocean swell which is dissipated rapidly as it propagates through the estuary mouth and is absent 1.5 km upstream. The small fetch of the harbour also means locally-generated wind waves are insignificant, generally b 1 m. Shore platforms occur in locations where deep-water waves can impact the bedrock and here surfaces up to 60 m wide are formed. They occur to an elevation of 3.5 m above mean sea level and are characterised by a vertical seaward cliff at least 1 m high, often N 2 m in the most exposed locations. All the platforms are semi-horizontal and appear to occur where a joint or bedding plane is coincident with intertidal elevations, although there is little correspondence with a particular tidal limit, with surfaces occurring at all levels between MLWS and MHWS. Several horizontal surfaces are found above the limit of modern tides and those at c. 1.5 and 2.1 m levels are likely to result from higher sea levels. Lithological structure and rock resistance therefore appear to be the primary determinants of platform morphology, with the most distinct development occurring where these planes are coincident with the intertidal zones where most erosion occurs. Some morphological inheritance from previous higher sea levels may also be found within the estuary. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The development of a shore platform is dependant on the balance between the geological conditions of the particular environment (e.g. rock strength and jointing) and forcing (e.g. waves and tides), as well as subaerial erosion processes. Shore platforms are most commonly found on the open coast where wave, tidal and subaerial weathering processes are energetic enough to exploit weaknesses within the lithology. Much debate has however occurred regarding the relative efficiency of each process with a scientific consensus yet to be reached (e.g. Stephenson and Kirk, 2000a,b; Trenhaile, 2005). In particular the relationship between the width of a shore platform and the wave energy that impacts the coast is particularly controversial (see reviews of Trenhaile, 1987; Sunamura, 1992), and the rocky coast of Sydney and southern New South Wales, Australia, is often cited as an example both proving and refuting any relationship (e.g. Jutson, 1939; Bird and Dent, 1966; Abrahams and Oak, 1975). The study of Bird and Dent (1966) inferred that platforms are best developed in areas that are sheltered from southerly wave activity by islands and headlands. Abrahams and Oak (1975) on the
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other hand empirically tested this idea on several headlands and found that in fact no consistent relationship between width and wave exposure occurred. All these sites investigated are directly exposed to deep-water ocean swell, however within the estuaries that occur along this coast shore platforms are also found, formed in the same bedrock as the open coast (Andrews, 1916; Hedley, 1924). The process environment of estuaries is highly variable with both the magnitude and relative dominance of wave and tidal energy changing over relatively short distances (Dalrymple et al., 1992). Where entrances to estuarine systems are narrow, open-ocean waves are often rapidly dissipated upstream and in some cases the tidal prism can also be reduced. In other settings where the tidal harmonics are in phase within coastal inlets, currents may be significantly enhanced between embayments causing rapid changes in the relative efficiency of erosive processes (Dalrymple and Choi, 2007). By examining the morphology of shore platforms through these environments it may therefore be possible to infer whether a linkage between wave exposure and platform shape occurs, through simply moving upstream from the estuary mouth. Such a linkage could be especially apparent in environments where other variables which will influence erosion such as lithology and tides remain spatially constant. This study therefore sets out to investigate the morphological variation of estuarine shore platforms found in Sydney Harbour, Australia, a fetch-limited drowned river valley. By examining the
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variation in shore platform morphology within the harbour this paper sets out to examine the correspondence between these features and wave exposure. In addition it aims to compare this morphology to the often cited research results from the open ocean coast and thereby infer whether wave exposure or other influences, such as geology, dominate shore platform development in this microtidal estuarine setting. 2. Regional setting Middle Harbour, Sydney is a 10 km long drowned river valley-type estuary, up to 20 m deep, which along with the North Harbour and Port Jackson arms form the larger estuary of Sydney Harbour (Fig. 1). It is microtidal throughout with a mean spring tidal range of 1.3 m. Mean deep water ocean wave height at the entrance to Sydney Harbour is 1.6 m (T = 10 s) (Short and Trenaman, 1992), while maximum estimated height of locally-generated wind waves is 0.5 m within the estuary (Irvine, 1980). Little scientific investigation has been undertaken on the dissipation of deep-water ocean waves within the study area. Observations of locals, resident for N40 years, suggest they do not propagate further than Chinamans Beach (Fig. 1). The catchment rises to a maximum elevation of c. 200 m and is composed of the Triassic Hawkesbury Sandstone. This was deposited as a large craton-sourced braided river system (Miall and Jones, 2003) and has an average composition of 68% quartz and 20% clay with minor amounts of feldspar, mica and rock fragments (Herbert, 1983a).
Fig. 1. Location of Middle Harbour, Sydney, Australia, showing the width and location of the surveyed profiles along with variation in water depth through the study site. The platform widths as represented on the map are to scale (×10) for illustrative purposes. Bathymetry based on NSW Maritime, Middle and North Harbour Map, 2006.
Two broad depositional units occur; sheet and massive facies (Conaghan and Jones, 1975). The sheet facies is characterised by cosets of cross strata, with an average dip of 34° bounded by planar horizontal surfaces which range in thickness from a few centimetres to 5 m or more (Conaghan and Jones, 1975). The massive facies is a structureless to faintly parallel-stratified sandstone, where the laminae may extend laterally for up to 5 m without break (Jones and Rust, 1983). Vertical and subvertical joints are common, spaced from 0.3–10.0 m apart. Two dominant orientations occur, the main being 090–125°T, and a less prominent 005–035°T (Herbert, 1983b). The region is tectonically stable, with no uplift occurring during the Quaternary (Bryant, 1992). Sea level reached close to present elevations by 7.5 ka and is most likely to have been 1 m higher 2 ka (Sloss et al., 2007) before falling to present levels.
3. Materials and methods Surveying was undertaken in January 2007 using a SOKKIA SET 4010 electronic distance metre (EDM). A total of 13 representative profiles were surveyed, spaced at regular intervals along the accessible rocky shoreline of Middle Harbour where platforms are found. Surveying was conducted in Middle Harbour as this arm of Sydney Harbour faces directly east into the Tasman Sea, unlike the Port Jackson and North Harbour sections where waves have to refract in a southerly and northerly direction respectively, before entering these arms (Fig. 1). Platforms do occur in the other arms of the harbour, however, accessibility and time limits prohibited field surveying of these locations. Profiles were measured from the cliff-platform junction to the seaward edge, marked either by burial by sand or a seaward cliff plunging below low tide level. All profiles were adjusted to mean sea level by applying a tidal correction to the still water level observed during the survey. The field surveys were complemented with an analysis of aerial photography (Sydney, 19/9/91 colour, NSW4038, 04). The seaward edge of the platforms is defined on the basis of the colour change between the platforms and water and the presence of wave swash. The landward edge is more difficult to define as the platforms close to mean sea level are often backed by higher horizontal bedding surfaces cut by joint planes. These form a series of steps along the coast which are difficult to distinguish from the contemporary platform. Generally the platform/vegetation boundary is taken as the landward edge, although where large boulders are present on the platform rear the landward edge is defined as the seaward limit of these boulders. To test the accuracy of this analysis these measurements were compared with the calculation of width derived from the EDM surveys for Grotto Point, Washaway, and Edwards Beach. An error of generally 2–5 m was noted between profiles, with the field surveys being 5–10% longer than those calculated from the photos. Shading on south facing platforms hindered analysis for some sites. Rock hardness readings (541) were taken across the platforms using an N-type Schmidt Hammer in accordance with the recommendation of Day and Goudie (1977) and Selby (1980). This was conducted in order to assess the total resistance of the platforms and the variation in compressive strength with elevation. This allows comparison with other workers results to be made and also to assess whether a decrease in rock strength resulting from subaerial weathering above MSL occurs. All surfaces were prepared with a carborundum wheel prior to testing with a minimum of 10 readings taken at each site. Data were subsequently corrected for those readings where the hammer was held away from the horizontal (Day and Goudie, 1977) and anomalously low values rejected on the basis of Chauvenet's criterion, a statistical method for removing spurious data (Göktan and Ayday, 1993). Only low values were subject to this test as it is mechanically impossible to obtain an erroneously high value (Göktan and Ayday, 1993).
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Fig. 2. The morphology of the surveyed shore platforms showing the sedimentary facies they are formed within.
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4. Results 4.1. Platform width Well developed shore platforms ranging up to 60 m wide (average 24.4 ± 17.6 m) occur throughout the study area in those areas that are exposed to deep-water ocean waves (Fig. 2). The widest platform (59.7 m) is found on the point at the southern end of Edwards Beach (Edwards4), facing due east towards the harbour entrance (Fig. 3a). Width then decreases in a northerly direction to 44.7 m at Edwards3 (second widest platform) and 17 m on Edwards1 (Fig. 3b). Beyond this profile the rocky shoreline is composed of vertical cliffs and boulder
slopes. The tip of Grotto Point (Grotto1) is also wide at 44.0 m, however this decreases to 19.0 m, 15 m to the east (Grotto2). Plunging cliffs are then found between Grotto Point and the Washaway profile to the northeast which is 36.4 m wide (Fig. 3c, d). Visual observations in the field suggest that the Washaway profile experiences the highest energy of those surveyed, as it is closest to, and directly faces, the open ocean. The narrowest platforms on the other hand are found along Castle Rock Beach where platform widths range from 6.9–10.6 m (Figs. 2 and 3e). Using aerial imagery analysis, it is possible to analyse platform width over a larger area than was accessible during field work. Although accurate for the surveyed sites, delineation of the rear of the
Fig. 3. (a) The + 1.5 m level on the Edwards4 profile. A small rampart 0.2 m high occurs on the seaward part of this platform, with water layer weathering pits landward. (b) The seaward portion of the Edwards2 profile occurring at MHWS level. Note the pitted nature of the seaward ramp and lower rear where water may pond. Preferential erosion along joints truncates the seaward edge. (c) Seaward edge of Grotto Point1 profile, showing imbricated boulders at its edge. (d) Stepped shore platform profile at Washaway. (e) Castle Rock Profile 1 and 2. The higher (profile 1) and lower (profile 2) levels are formed in a cross bedded unit. (f) Edwards 3 profile showing iron precipitation and the joints in the fore ground.
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contemporary platforms was difficult. As a result these data are presented in width classes rather than exact numbers. The majority of platforms within the study area are less than 20 m wide (Fig. 4a), their seaward edge commonly being delineated by a joint plane oriented between 010–030° T (Fig. 4b). The widest platforms are found on points, with width generally being less along straight sections of rocky shore. An exception to this occurs along Dobroyd Head where a section of platform 31–40 m wide occurs in the centre of the headland. Plunging cliff lines appear to occur only at those sites that directly face the open ocean, though as on Middle Head these sections are often flanked by shore platforms. In platform the top of the seaward cliff is very distinct, especially when the coast is orientated in a NNE direction, such as along Dobroyd Head where it forms a straight edge 50 m long (Fig. 4b). This morphology is directly related to the orientation of the joint planes which cross the platforms and form the seaward cliff. 4.2. Platform elevation The surveyed platforms in Middle Harbour occur at a range of elevations from − 0.7 to +6.1 m above mean sea level (MSL) (Fig. 2). The highest platform is found at Washaway Beach and it is characterised by a stepped profile with horizontal surfaces at 6.1, 3.5, 2.3 and 1.0 m above MSL, the lowest being subjected to contemporary wave processes. The profile shape is directly related to alternating massive and sheet facies within the sandstone, although the horizontal surfaces do not appear to be preferentially formed in a particular unit. This stepped morphology is characteristic of all the platforms within the study. All the profiles also have a vertical seaward cliff that plunges below MSL. At Castle Rock this seaward cliff is buried by sandy beaches, while at the other locations it is at least 1 m high, although the cliff/sea floor junction was not observed. The widest horizontal section at Washaway (15.8 m) occurs at +1.0 m, although at +3.5 m it is only slightly smaller at 13.4 m. On this
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latter part of the profile a ramp 0.2 m high occurs at its seaward edge, although such a feature is not found at the + 1.0 m level. At the tip of Grotto Point (profile Grotto2) a similar horizontal section is found at +3.5 m and is truncated at its landward edge by a channel 1.5 m deep which has eroded along a vertical joint oriented east–west. A smaller horizontal surface occurs between +2.2 and + 2.3 m on both of these platforms, 2.0 and 4.8 m wide on Washaway and Grotto2 respectively (Fig. 3d). A surface at this elevation can also be found further upstream in the harbour on Edwards4, where it is 3.4 m wide. On Edwards4 a surface 22.6 m wide occurs at an elevation of +1.5 m which is occasionally affected by storm-wave splash (Fig. 3a). Surfaces at a similar elevation are also found at Castle Rock between +1.4–+1.9 m. On the lower of these (CastleRk2 profile) sand from an adjacent beach has been observed to accumulate (Fig. 3e). Within the intertidal zone, platforms can be observed at all locations at an elevation of + 0.63 m (MHWS)–+ 1.0 m. On Washaway, the lowest platform (15.7 m wide) occurs at +1.0 m, is covered with macroalgae, and constantly swept by waves (Fig. 3d). The same level occurs on Edwards3, while the widest horizontal surface (5.9 m) on the narrow Castle Rock profiles occurs at MHWS level. A distinct level of erosion appears to occur at MSL, being widest on Grotto1, but also present on the Edwards Beach profiles. The lowest profile surveyed occurs on Grotto Point at approximate MLWS elevation (−0.63 m), which also corresponds to the maximum depth of incision of the profiles on Edwards Beach (Fig. 2). 4.3. Platform hardness The mean Schmidt hammer rebound values for the Middle Harbour platforms is 38.9 ± 6.2, with a range of 29–60 (Fig. 5). Both the massive and sheet facies are found to form the various horizontal surfaces in the sandstone however little distinct variation in hardness values is found between these units (Fig. 5). Secondary limonite (iron) staining is found throughout the sandstone and it is especially concentrated along bedding and joint planes (Fig. 3f). In some locations, such as on
Fig. 4. (a) Shore platform widths from Middle and North Harbours calculated from aerial photography. (b) Aerial image of the platform along Dobroyd Head. Note the lineation in the seaward edges of the platform delineated by vertical joint planes. Image reproduced with kind permission of Google™.
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Fig. 5. Variation in mean N-type Schmidt Hammer rebound values (± 1σ) along the surveyed platforms. Little relation exists between platform hardness and elevation above MSL.
the seaward edge of the Edwards3 profile, a layer several centimetres thick forms, and it is at these locations that the highest rebound values (r N 47) are recorded. Hardness readings were also taken from all horizontal surfaces found on the surveyed profiles, from +0.1 m on Grotto Point (profile 2) to +7.5 m above MSL on the Washaway profile. Little relationship occurs between the elevation of the readings and the Schmidt hammer rebound values, with an almost random correlation being observed (R2 = 0.01). For example in the massive facies at 7.5 m elevation on Washaway rebound a value of 41 is found, the same as at 1.4 m on Castle Rock3, and just higher than a rebound value of 39 at 0.5 m on Edwards4. The hardness values for all the platforms and facies are therefore very similar, except where secondary iron minerals such as limonite have been precipitated (Fig. 3f). 5. Discussion Shore platforms are found throughout the study area, developed in Triassic sandstones, in those locations where deep-water wave energy can impact the shoreline. Little scientific surveying has been conducted on the open-ocean shore platforms of Sydney developed within the same lithology, however the descriptions of Hedley (1924) and Bird and Dent (1966), suggest they are wider, up to 60 m. This is similar to the width of platforms surveyed in sandstone units south of Sydney (Abrahams and Oak, 1975). Personal observations of the platforms along the Sydney coast confirm this with platform width being highly variable related to the joint orientation and may range from a plunging cliff to several 10's of metres wide over a short distance (b50 m). In Middle Harbour, apart from the correspondence between the location of platforms and where deep-water wave energy can impact the coast, a direct relationship between width and energy does not appear to occur. While wave energy was not directly measured it is possible to use the depth immediately offshore of a platform as a surrogate, as has been done in Japan (Sunamura, 1992), and the Tasman Sea (Dickson et al., 2004; Dickson 2006). On all the platforms surveyed the seaward cliff plunges to over 2 m water depth, with the exception of Castle Rock, where the seaward cliff is buried by sand to an elevation close to MSL (Fig. 3e). Bathymetry is however relatively consistent immediately off the surveyed platforms, dropping to 5 m depth several metres from the seaward cliff, except at Castle Rock where a sandy bank extends 300 m offshore. Interestingly, on the plunging cliff lines directly north of Washaway Beach, water depths are between 5 and 10 m, however on Dobroyd Point, with a similar nearshore depth, platforms up to 30 m wide occur (Figs. 1 and 4).
The marked seaward edge of many of the platforms described in this study corresponds to the joint planes, suggesting that lithology may have an overriding influence on platform width and therefore overall morphology. The ability of erosive process to overcome the resistance of the rock mass therefore is important. Few studies have been conducted in estuarine and bay environments and those which have generally occur in low-energy areas such as in Whanganui Inlet (Kennedy and Paulik, 2007) or Auckland, New Zealand (de Lange and Moon, 2005). Wave energy at the shoreline in these locations is generally b1 m and dominated by fetch-limited locally generated wind waves. Platforms do however occur in these locations, and in Whanganui Inlet platforms over 100 m wide are found, forming in fine sandstones with Schmidt hammer (N-type) rebound values of 17 ± 8 (Kennedy and Paulik, 2007). In this area where erosive energy is high, such as adjacent to tidal channels several metres deep, the platforms are completely eroded and plunging cliffs occur. Comparisons with Middle Harbour are therefore difficult as these systems occur in quite a different process regime and much softer lithologies. Rock structure, in addition to strength, will influence its vulnerability to the erosive effect of waves (Trenhaile, 2004). For example on Shag Point, New Zealand, joint sets in fine sandstones (R = 31 ± 4), causes platforms to form at low tide elevations while unjointed platforms of the same hardness are found at high tide level (Kennedy and Dickson, 2006). These joints are spaced c. 0.5 m apart and are filled with iron oxidation. In areas where joints are open, waves can be very efficient at removing blocks (Trenhaile and Kanyaya, 2007), with wave quarrying occurring along these planes (Bird and Dent, 1966). In Wellington, New Zealand, high densities of open joints (N10/m) can cause lateral erosion rates of up to 0.1 m/year (Kennedy and Beban, 2005). The vertical joints in the Hawkesbury Sandstone in Middle Harbour are generally several metres apart and often filled with iron oxidation. Erosion appears to preferentially occur along these planes, however as the joints are most often spaced many metres apart, and combined with beds over a metre thick, wave quarrying appears limited due to the size of the resulting blocks. This is then reflected in the linear nature of the seaward cliff of the platforms which is defined by the joint sets (Fig. 4b). This finding is similar to the open-ocean coast of Sydney where the structural control was found to be more important than wave intensity (Abrahams and Oak, 1975), allowing platforms to only form where variations in lithology occur (Bird and Dent, 1966). The resistant nature of the Hawkesbury Sandstone as indicated by its compressive strength, massive bedding and widely spaced joint planes, means all the platforms, except Grotto3 at MLWS, have formed at or above MSL. Many in fact rise several metres above the limit of modern marine processes which suggests that erosion may not entirely relate to contemporary processes. Erosion during previous sea level stands often affects the morphology of contemporary platforms (Trenhaile, 2002) especially where the rock resistance may preserve terraces over glacial/ interglacial cycles, such as occurs in Spain (Blancho Chao et al., 2003) and Lord Howe Island, Australia (Dickson, 2006). Along the NSW coast this is an important issue (Stephenson and Thornton, 2005), with the possible inherited nature of platforms in Sydney being debated almost a century ago (Andrews, 1916; Hedley, 1924; Jutson, 1939). In southern NSW elevated platforms have been related to higher sea levels from the previous two interglacials (Bryant et al., 1992; Brooke et al., 1994), and based on surveys of highstand shorelines around Australia these higher sea levels would also have occurred within Middle Harbour (Murray-Wallace and Belperio, 1991; Bryant, 1992). Within the active marine zone in Middle Harbour platform elevations are generally close to MSL although ranging between MLWS and MHWS, with the highest platform within the modern wave zone being found at Washaway Beach at an elevation of +1 m and covered in macroalgae. It is therefore likely that this range would also occur for those platforms developing at a different sea
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level. A distinct horizontal surface is found at Edwards4 over 20 m wide at 1.5 m above MSL and has a relict rampart and possible former water layer weathering pits (Fig. 3a). A similar level occurs at Grotto2 (+1.4 m) and appears to correspond to erosion along a small crossbedded unit (Fig. 3e). A more distinct and spatially continuous surface is found at around +2.2 m. This occurs on the Washaway, Grotto and Edwards Beach sections and may also be represented by a slightly lower surface at + 2 m on Castle Rock1. These 1.5 and 2.2 m surfaces were ascribed to previous levels of marine erosion (Hedley, 1924). A consistent raised platform level of +2 m also occurs along the southern NSW coast and has been dated at the Last Interglacial (LIG) between 80 and 105 ka (Young and Bryant, 1993; Brooke et al., 1994). Young and Bryant (1993) also attribute a higher surface at +4 m to earlier during the LIG at 120–140 ka it is tempting to relate this level to the platforms at + 3.5 m on the Grotto2 and Washaway profiles. It therefore appears most likely that the morphology of the platforms within Middle Harbour is partly inherited from erosive processes during past higher sea levels dating back to the Last Interglacial Period. 6. Conclusions Shore platforms within Middle Harbour, Sydney, form where deepwater ocean waves can impact the rocky shoreline. Joint planes within the sandstone are widely spaced and the massive nature of the beds, combined with their compressive strength of 39 ± 6 means they are relatively resistant to erosion. Near vertical joints appear to characterise the seaward edge of the platforms forming low-tide cliffs. Erosion within the intertidal zones appears to preferentially occur along bedding planes and where these correspond to the intertidal zone, wide platforms are formed. The resistant nature of the lithology means that several platform levels derived from previous higher sea levels occur, namely at c. 1.5–2 and 3.5 m above MSL. Shore platform morphology within Middle Harbour therefore appears to be primarily determined by geological structure, with marine processes only able to exploit existing planes of weakness located within the intertidal zone. Acknowledgements The author would like to thank Michael Kennedy for his assistance in the field. This paper is a contribution to the IAG/AIG working group on Rocky Coasts. References Abrahams, A.D., Oak, H.L., 1975. Shore platform widths between Port Kembla and Durras Lake, New South Wales. Australian Geographical Studies 13, 190–194. Andrews, E.C., 1916. Shoreline studies at Botany Bay. Journal of the Royal Society of New South Wales 50, 165–176. Bird, E.C.F., Dent, O.F., 1966. Shore platforms on the south coast of New South Wales. Australian Geographer 10, 71–80. Blancho Chao, R.B., Costa Casais, M.C., Martinez Cortizas, A.M., Perez Alberti, A.P., Trenhaile, A.S., 2003. Evolution and inheritance of a rock coast: Western Galicia, Northwestern Spain. Earth Surface Processes and Landforms 28, 757–775. Brooke, B.P., Young, R.W., Bryant, E.A., Murray-Wallace, C.V., Price, D.M., 1994. A Pleistocene origin for shore platforms along the northern Illawarra coast, New South Wales. Australian Geographer 25, 178–185. Bryant, E.A., 1992. Late Interglacial and Holocene trends in sea-level maxima around Australia: implications for modern rates. Marine Geology 108, 209–217. Bryant, E.A., Young, R.W., Price, D.M., Short, S.A., 1992. Evidence for Pleistocene and Holocene raised marine deposits, Sandon Point, New South Wales. Australian Journal of Earth Sciences 39, 481–494. Conaghan, P.J., Jones, J.G., 1975. The Hawkesbury Sandstone and the Brahmaputra: a depositional model for continental sheet sandstones. Journal of the Geological Society of Australia 22, 275–283.
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Dalrymple, R.W., Choi, K., 2007. Morphologic and facies trends through the fluvial– marine transition in tide-dominated depositional systems: a schematic framework for environmental and sequence-stratigraphic interpretation. Earth Science Reviews 81, 135–174. Dalrymple, R.W., Zaitlin, B.A., Boyd, R., 1992. Estuarine facies models: conceptual basis and stratigraphic implications. Journal of Sedimentary Petrology 62, 1130–1146. Day, M.J., Goudie, A.S., 1977. Field assessment of rock hardness using the Schmidt test hammer. British Geomorphological Research Group Technical Bulletin 18, 19–29. de Lange, W.P., Moon, V.G., 2005. Estimating long-term cliff recession rates from shore platform widths. Engineering Geology 80, 292–301. Dickson, M.E., 2006. Shore platforms development around Lord Howe Island, southwest Pacific. Geomorphology 76, 295–315. Dickson, M.E., Kennedy, D.M., Woodroffe, C.D., 2004. The influence of rock resistance on coastal morphology around Lord Howe Island, southwest Pacific. Earth Surface Processes and Landforms 29, 629–643. Göktan, R.M., Ayday, C., 1993. A suggested improvement to the Schmidt Rebound Hardness ISRM suggested method with particular reference to rock machineability. International Journal of Rock Mechanics 30, 321–322. Hedley, C., 1924. Differential elevation near Sydney. Journal of the Royal Society of New South Wales 58, 61–69. Herbert, C., 1983a. Sydney Basin stratigraphy. In: Herbert, C. (Ed.), Geology of the Sydney 1:100,000 Sheet 9130. New South Wales Department of Mineral Resources, Sydney, pp. 7–34. Herbert, C., 1983b. Structural geology. In: Herbert, C. (Ed.), Geology of the Sydney 1:100,000 Sheet 9130. New South Wales Department of Mineral Resources, Sydney, pp. 115–119. Irvine, I., 1980. Sydney Harbour sediments and heavy metal pollution. PhD Thesis, University of Sydney, 260 pp. Jones, B.G., Rust, B.R., 1983. Massive sandstone facies in the Hawkesbury Sandstone, a Triassic fluvial deposit near Sydney, Australia. Journal of Sedimentary Petrology 53, 1249–1259. Jutson, J.T., 1939. Shore platforms near Sydney, New South Wales. Journal of Geomorphology 2, 237–250. Kennedy, D.M., Beban, J.G., 2005. Shore platform morphology on a rapidly uplifting coast, Wellington, New Zealand. Earth Surface Processes and Landforms 30, 823–832. Kennedy, D.M., Dickson, M.E., 2006. Lithological control on the elevation of shore platforms in a microtidal setting. Earth Surface Processes and Landforms 31, 1575–1584. Kennedy, D.M., Paulik, R., 2007. Estuarine platforms in Whanganui Inlet, South Island, New Zealand. Geomorphology 88, 214–225. Miall, A.D., Jones, B.G., 2003. Fluvial architecture of the Hawkesbury Sandstone (Triassic), near Sydney, Australia. Journal of Sedimentary Research 73, 531–545. Murray-Wallace, C.V., Belperio, A.P., 1991. The Last Interglacial shoreline in Australia—a Review. Quaternary Science Reviews 10, 441–461. Selby, M.J., 1980. A rock mass strength classification for geomorphic purposes with tests from Antarctica and New Zealand. Zeitschrift für Geomorphologie 24, 31–51. Short, A., Trenaman, N., 1992. Wave climate of the Sydney region, an energetic and highly variable ocean wave regime. Australian Journal of Marine and Freshwater Research 43, 765–791. Sloss, C.R., Murray-Wallace, C.V., Jones, B.G., 2007. Holocene sea-level change on the southeast coast of Australia: a review. The Holocene 17, 1001–1016. Stephenson, W.J., Kirk, R.M., 2000a. Development of shore platforms on Kaikoura Peninsula, South Island, New Zealand part one: the role of waves. Geomorphology 32, 21–41. Stephenson, W.J., Kirk, R.M., 2000b. Development of shore platforms on Kaikoura Peninsula, South Island, New Zealand II: the role of subaerial weathering. Geomorphology 32, 43–56. Stephenson, W.J., Thornton, L.E., 2005. Australian rock coasts: review and prospects. Australian Geographer 36, 95–115. Sunamura, T., 1992. Geomorphology of rocky coasts. Wiley, Chichester. 302 pp. Trenhaile, A.S., 1987. The geomorphology of rocky coasts. Clarendon Press, Oxford. 366 pp. Trenhaile, A.S., 2002. Rock coasts, with particular emphasis on shore platforms. Geomorphology 48, 7–22. Trenhaile, A.S., 2004. Modeling the effect of tidal wetting and drying on shore platform development. Journal of Coastal Research 20, 1049–1060. Trenhaile, A.S., 2005. Modelling the effect of waves, weathering and beach development on shore platform development. Earth Surface Processes and Landforms 30, 613–634. Trenhaile, A.S., Kanyaya, J.I., 2007. The role of wave erosion on sloping and horizontal shore platforms in macro- and mesotidal environments. Journal of Coastal Research 23, 298–309. Young, R.W., Bryant, E.A., 1993. Coastal rock platforms and ramps of Pleistocene and Tertiary age in Southern New South Wales. Zeitschrift für Geomorphologie 37, 257–272.
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Geological control on the morphology of estuarine shore platforms: Middle Harbour, Sydney, Australia D.M. Kennedy ⁎ School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand
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Article history: Accepted 5 February 2009 Available online 15 February 2009 Keywords: Shore platform Estuary Erosion Joint Sandstone Rock coast Inheritance
a b s t r a c t Middle Harbour, Sydney, Australia is a microtidal environment formed in resistant Triassic quartzose sandstone (mean N-type Schmidt Hammer rebound value of 37 ± 6). Wave energy is dominated by deepwater ocean swell which is dissipated rapidly as it propagates through the estuary mouth and is absent 1.5 km upstream. The small fetch of the harbour also means locally-generated wind waves are insignificant, generally b 1 m. Shore platforms occur in locations where deep-water waves can impact the bedrock and here surfaces up to 60 m wide are formed. They occur to an elevation of 3.5 m above mean sea level and are characterised by a vertical seaward cliff at least 1 m high, often N 2 m in the most exposed locations. All the platforms are semi-horizontal and appear to occur where a joint or bedding plane is coincident with intertidal elevations, although there is little correspondence with a particular tidal limit, with surfaces occurring at all levels between MLWS and MHWS. Several horizontal surfaces are found above the limit of modern tides and those at c. 1.5 and 2.1 m levels are likely to result from higher sea levels. Lithological structure and rock resistance therefore appear to be the primary determinants of platform morphology, with the most distinct development occurring where these planes are coincident with the intertidal zones where most erosion occurs. Some morphological inheritance from previous higher sea levels may also be found within the estuary. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The development of a shore platform is dependant on the balance between the geological conditions of the particular environment (e.g. rock strength and jointing) and forcing (e.g. waves and tides), as well as subaerial erosion processes. Shore platforms are most commonly found on the open coast where wave, tidal and subaerial weathering processes are energetic enough to exploit weaknesses within the lithology. Much debate has however occurred regarding the relative efficiency of each process with a scientific consensus yet to be reached (e.g. Stephenson and Kirk, 2000a,b; Trenhaile, 2005). In particular the relationship between the width of a shore platform and the wave energy that impacts the coast is particularly controversial (see reviews of Trenhaile, 1987; Sunamura, 1992), and the rocky coast of Sydney and southern New South Wales, Australia, is often cited as an example both proving and refuting any relationship (e.g. Jutson, 1939; Bird and Dent, 1966; Abrahams and Oak, 1975). The study of Bird and Dent (1966) inferred that platforms are best developed in areas that are sheltered from southerly wave activity by islands and headlands. Abrahams and Oak (1975) on the
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other hand empirically tested this idea on several headlands and found that in fact no consistent relationship between width and wave exposure occurred. All these sites investigated are directly exposed to deep-water ocean swell, however within the estuaries that occur along this coast shore platforms are also found, formed in the same bedrock as the open coast (Andrews, 1916; Hedley, 1924). The process environment of estuaries is highly variable with both the magnitude and relative dominance of wave and tidal energy changing over relatively short distances (Dalrymple et al., 1992). Where entrances to estuarine systems are narrow, open-ocean waves are often rapidly dissipated upstream and in some cases the tidal prism can also be reduced. In other settings where the tidal harmonics are in phase within coastal inlets, currents may be significantly enhanced between embayments causing rapid changes in the relative efficiency of erosive processes (Dalrymple and Choi, 2007). By examining the morphology of shore platforms through these environments it may therefore be possible to infer whether a linkage between wave exposure and platform shape occurs, through simply moving upstream from the estuary mouth. Such a linkage could be especially apparent in environments where other variables which will influence erosion such as lithology and tides remain spatially constant. This study therefore sets out to investigate the morphological variation of estuarine shore platforms found in Sydney Harbour, Australia, a fetch-limited drowned river valley. By examining the
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variation in shore platform morphology within the harbour this paper sets out to examine the correspondence between these features and wave exposure. In addition it aims to compare this morphology to the often cited research results from the open ocean coast and thereby infer whether wave exposure or other influences, such as geology, dominate shore platform development in this microtidal estuarine setting. 2. Regional setting Middle Harbour, Sydney is a 10 km long drowned river valley-type estuary, up to 20 m deep, which along with the North Harbour and Port Jackson arms form the larger estuary of Sydney Harbour (Fig. 1). It is microtidal throughout with a mean spring tidal range of 1.3 m. Mean deep water ocean wave height at the entrance to Sydney Harbour is 1.6 m (T = 10 s) (Short and Trenaman, 1992), while maximum estimated height of locally-generated wind waves is 0.5 m within the estuary (Irvine, 1980). Little scientific investigation has been undertaken on the dissipation of deep-water ocean waves within the study area. Observations of locals, resident for N40 years, suggest they do not propagate further than Chinamans Beach (Fig. 1). The catchment rises to a maximum elevation of c. 200 m and is composed of the Triassic Hawkesbury Sandstone. This was deposited as a large craton-sourced braided river system (Miall and Jones, 2003) and has an average composition of 68% quartz and 20% clay with minor amounts of feldspar, mica and rock fragments (Herbert, 1983a).
Fig. 1. Location of Middle Harbour, Sydney, Australia, showing the width and location of the surveyed profiles along with variation in water depth through the study site. The platform widths as represented on the map are to scale (×10) for illustrative purposes. Bathymetry based on NSW Maritime, Middle and North Harbour Map, 2006.
Two broad depositional units occur; sheet and massive facies (Conaghan and Jones, 1975). The sheet facies is characterised by cosets of cross strata, with an average dip of 34° bounded by planar horizontal surfaces which range in thickness from a few centimetres to 5 m or more (Conaghan and Jones, 1975). The massive facies is a structureless to faintly parallel-stratified sandstone, where the laminae may extend laterally for up to 5 m without break (Jones and Rust, 1983). Vertical and subvertical joints are common, spaced from 0.3–10.0 m apart. Two dominant orientations occur, the main being 090–125°T, and a less prominent 005–035°T (Herbert, 1983b). The region is tectonically stable, with no uplift occurring during the Quaternary (Bryant, 1992). Sea level reached close to present elevations by 7.5 ka and is most likely to have been 1 m higher 2 ka (Sloss et al., 2007) before falling to present levels.
3. Materials and methods Surveying was undertaken in January 2007 using a SOKKIA SET 4010 electronic distance metre (EDM). A total of 13 representative profiles were surveyed, spaced at regular intervals along the accessible rocky shoreline of Middle Harbour where platforms are found. Surveying was conducted in Middle Harbour as this arm of Sydney Harbour faces directly east into the Tasman Sea, unlike the Port Jackson and North Harbour sections where waves have to refract in a southerly and northerly direction respectively, before entering these arms (Fig. 1). Platforms do occur in the other arms of the harbour, however, accessibility and time limits prohibited field surveying of these locations. Profiles were measured from the cliff-platform junction to the seaward edge, marked either by burial by sand or a seaward cliff plunging below low tide level. All profiles were adjusted to mean sea level by applying a tidal correction to the still water level observed during the survey. The field surveys were complemented with an analysis of aerial photography (Sydney, 19/9/91 colour, NSW4038, 04). The seaward edge of the platforms is defined on the basis of the colour change between the platforms and water and the presence of wave swash. The landward edge is more difficult to define as the platforms close to mean sea level are often backed by higher horizontal bedding surfaces cut by joint planes. These form a series of steps along the coast which are difficult to distinguish from the contemporary platform. Generally the platform/vegetation boundary is taken as the landward edge, although where large boulders are present on the platform rear the landward edge is defined as the seaward limit of these boulders. To test the accuracy of this analysis these measurements were compared with the calculation of width derived from the EDM surveys for Grotto Point, Washaway, and Edwards Beach. An error of generally 2–5 m was noted between profiles, with the field surveys being 5–10% longer than those calculated from the photos. Shading on south facing platforms hindered analysis for some sites. Rock hardness readings (541) were taken across the platforms using an N-type Schmidt Hammer in accordance with the recommendation of Day and Goudie (1977) and Selby (1980). This was conducted in order to assess the total resistance of the platforms and the variation in compressive strength with elevation. This allows comparison with other workers results to be made and also to assess whether a decrease in rock strength resulting from subaerial weathering above MSL occurs. All surfaces were prepared with a carborundum wheel prior to testing with a minimum of 10 readings taken at each site. Data were subsequently corrected for those readings where the hammer was held away from the horizontal (Day and Goudie, 1977) and anomalously low values rejected on the basis of Chauvenet's criterion, a statistical method for removing spurious data (Göktan and Ayday, 1993). Only low values were subject to this test as it is mechanically impossible to obtain an erroneously high value (Göktan and Ayday, 1993).
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Fig. 2. The morphology of the surveyed shore platforms showing the sedimentary facies they are formed within.
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4. Results 4.1. Platform width Well developed shore platforms ranging up to 60 m wide (average 24.4 ± 17.6 m) occur throughout the study area in those areas that are exposed to deep-water ocean waves (Fig. 2). The widest platform (59.7 m) is found on the point at the southern end of Edwards Beach (Edwards4), facing due east towards the harbour entrance (Fig. 3a). Width then decreases in a northerly direction to 44.7 m at Edwards3 (second widest platform) and 17 m on Edwards1 (Fig. 3b). Beyond this profile the rocky shoreline is composed of vertical cliffs and boulder
slopes. The tip of Grotto Point (Grotto1) is also wide at 44.0 m, however this decreases to 19.0 m, 15 m to the east (Grotto2). Plunging cliffs are then found between Grotto Point and the Washaway profile to the northeast which is 36.4 m wide (Fig. 3c, d). Visual observations in the field suggest that the Washaway profile experiences the highest energy of those surveyed, as it is closest to, and directly faces, the open ocean. The narrowest platforms on the other hand are found along Castle Rock Beach where platform widths range from 6.9–10.6 m (Figs. 2 and 3e). Using aerial imagery analysis, it is possible to analyse platform width over a larger area than was accessible during field work. Although accurate for the surveyed sites, delineation of the rear of the
Fig. 3. (a) The + 1.5 m level on the Edwards4 profile. A small rampart 0.2 m high occurs on the seaward part of this platform, with water layer weathering pits landward. (b) The seaward portion of the Edwards2 profile occurring at MHWS level. Note the pitted nature of the seaward ramp and lower rear where water may pond. Preferential erosion along joints truncates the seaward edge. (c) Seaward edge of Grotto Point1 profile, showing imbricated boulders at its edge. (d) Stepped shore platform profile at Washaway. (e) Castle Rock Profile 1 and 2. The higher (profile 1) and lower (profile 2) levels are formed in a cross bedded unit. (f) Edwards 3 profile showing iron precipitation and the joints in the fore ground.
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contemporary platforms was difficult. As a result these data are presented in width classes rather than exact numbers. The majority of platforms within the study area are less than 20 m wide (Fig. 4a), their seaward edge commonly being delineated by a joint plane oriented between 010–030° T (Fig. 4b). The widest platforms are found on points, with width generally being less along straight sections of rocky shore. An exception to this occurs along Dobroyd Head where a section of platform 31–40 m wide occurs in the centre of the headland. Plunging cliff lines appear to occur only at those sites that directly face the open ocean, though as on Middle Head these sections are often flanked by shore platforms. In platform the top of the seaward cliff is very distinct, especially when the coast is orientated in a NNE direction, such as along Dobroyd Head where it forms a straight edge 50 m long (Fig. 4b). This morphology is directly related to the orientation of the joint planes which cross the platforms and form the seaward cliff. 4.2. Platform elevation The surveyed platforms in Middle Harbour occur at a range of elevations from − 0.7 to +6.1 m above mean sea level (MSL) (Fig. 2). The highest platform is found at Washaway Beach and it is characterised by a stepped profile with horizontal surfaces at 6.1, 3.5, 2.3 and 1.0 m above MSL, the lowest being subjected to contemporary wave processes. The profile shape is directly related to alternating massive and sheet facies within the sandstone, although the horizontal surfaces do not appear to be preferentially formed in a particular unit. This stepped morphology is characteristic of all the platforms within the study. All the profiles also have a vertical seaward cliff that plunges below MSL. At Castle Rock this seaward cliff is buried by sandy beaches, while at the other locations it is at least 1 m high, although the cliff/sea floor junction was not observed. The widest horizontal section at Washaway (15.8 m) occurs at +1.0 m, although at +3.5 m it is only slightly smaller at 13.4 m. On this
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latter part of the profile a ramp 0.2 m high occurs at its seaward edge, although such a feature is not found at the + 1.0 m level. At the tip of Grotto Point (profile Grotto2) a similar horizontal section is found at +3.5 m and is truncated at its landward edge by a channel 1.5 m deep which has eroded along a vertical joint oriented east–west. A smaller horizontal surface occurs between +2.2 and + 2.3 m on both of these platforms, 2.0 and 4.8 m wide on Washaway and Grotto2 respectively (Fig. 3d). A surface at this elevation can also be found further upstream in the harbour on Edwards4, where it is 3.4 m wide. On Edwards4 a surface 22.6 m wide occurs at an elevation of +1.5 m which is occasionally affected by storm-wave splash (Fig. 3a). Surfaces at a similar elevation are also found at Castle Rock between +1.4–+1.9 m. On the lower of these (CastleRk2 profile) sand from an adjacent beach has been observed to accumulate (Fig. 3e). Within the intertidal zone, platforms can be observed at all locations at an elevation of + 0.63 m (MHWS)–+ 1.0 m. On Washaway, the lowest platform (15.7 m wide) occurs at +1.0 m, is covered with macroalgae, and constantly swept by waves (Fig. 3d). The same level occurs on Edwards3, while the widest horizontal surface (5.9 m) on the narrow Castle Rock profiles occurs at MHWS level. A distinct level of erosion appears to occur at MSL, being widest on Grotto1, but also present on the Edwards Beach profiles. The lowest profile surveyed occurs on Grotto Point at approximate MLWS elevation (−0.63 m), which also corresponds to the maximum depth of incision of the profiles on Edwards Beach (Fig. 2). 4.3. Platform hardness The mean Schmidt hammer rebound values for the Middle Harbour platforms is 38.9 ± 6.2, with a range of 29–60 (Fig. 5). Both the massive and sheet facies are found to form the various horizontal surfaces in the sandstone however little distinct variation in hardness values is found between these units (Fig. 5). Secondary limonite (iron) staining is found throughout the sandstone and it is especially concentrated along bedding and joint planes (Fig. 3f). In some locations, such as on
Fig. 4. (a) Shore platform widths from Middle and North Harbours calculated from aerial photography. (b) Aerial image of the platform along Dobroyd Head. Note the lineation in the seaward edges of the platform delineated by vertical joint planes. Image reproduced with kind permission of Google™.
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Fig. 5. Variation in mean N-type Schmidt Hammer rebound values (± 1σ) along the surveyed platforms. Little relation exists between platform hardness and elevation above MSL.
the seaward edge of the Edwards3 profile, a layer several centimetres thick forms, and it is at these locations that the highest rebound values (r N 47) are recorded. Hardness readings were also taken from all horizontal surfaces found on the surveyed profiles, from +0.1 m on Grotto Point (profile 2) to +7.5 m above MSL on the Washaway profile. Little relationship occurs between the elevation of the readings and the Schmidt hammer rebound values, with an almost random correlation being observed (R2 = 0.01). For example in the massive facies at 7.5 m elevation on Washaway rebound a value of 41 is found, the same as at 1.4 m on Castle Rock3, and just higher than a rebound value of 39 at 0.5 m on Edwards4. The hardness values for all the platforms and facies are therefore very similar, except where secondary iron minerals such as limonite have been precipitated (Fig. 3f). 5. Discussion Shore platforms are found throughout the study area, developed in Triassic sandstones, in those locations where deep-water wave energy can impact the shoreline. Little scientific surveying has been conducted on the open-ocean shore platforms of Sydney developed within the same lithology, however the descriptions of Hedley (1924) and Bird and Dent (1966), suggest they are wider, up to 60 m. This is similar to the width of platforms surveyed in sandstone units south of Sydney (Abrahams and Oak, 1975). Personal observations of the platforms along the Sydney coast confirm this with platform width being highly variable related to the joint orientation and may range from a plunging cliff to several 10's of metres wide over a short distance (b50 m). In Middle Harbour, apart from the correspondence between the location of platforms and where deep-water wave energy can impact the coast, a direct relationship between width and energy does not appear to occur. While wave energy was not directly measured it is possible to use the depth immediately offshore of a platform as a surrogate, as has been done in Japan (Sunamura, 1992), and the Tasman Sea (Dickson et al., 2004; Dickson 2006). On all the platforms surveyed the seaward cliff plunges to over 2 m water depth, with the exception of Castle Rock, where the seaward cliff is buried by sand to an elevation close to MSL (Fig. 3e). Bathymetry is however relatively consistent immediately off the surveyed platforms, dropping to 5 m depth several metres from the seaward cliff, except at Castle Rock where a sandy bank extends 300 m offshore. Interestingly, on the plunging cliff lines directly north of Washaway Beach, water depths are between 5 and 10 m, however on Dobroyd Point, with a similar nearshore depth, platforms up to 30 m wide occur (Figs. 1 and 4).
The marked seaward edge of many of the platforms described in this study corresponds to the joint planes, suggesting that lithology may have an overriding influence on platform width and therefore overall morphology. The ability of erosive process to overcome the resistance of the rock mass therefore is important. Few studies have been conducted in estuarine and bay environments and those which have generally occur in low-energy areas such as in Whanganui Inlet (Kennedy and Paulik, 2007) or Auckland, New Zealand (de Lange and Moon, 2005). Wave energy at the shoreline in these locations is generally b1 m and dominated by fetch-limited locally generated wind waves. Platforms do however occur in these locations, and in Whanganui Inlet platforms over 100 m wide are found, forming in fine sandstones with Schmidt hammer (N-type) rebound values of 17 ± 8 (Kennedy and Paulik, 2007). In this area where erosive energy is high, such as adjacent to tidal channels several metres deep, the platforms are completely eroded and plunging cliffs occur. Comparisons with Middle Harbour are therefore difficult as these systems occur in quite a different process regime and much softer lithologies. Rock structure, in addition to strength, will influence its vulnerability to the erosive effect of waves (Trenhaile, 2004). For example on Shag Point, New Zealand, joint sets in fine sandstones (R = 31 ± 4), causes platforms to form at low tide elevations while unjointed platforms of the same hardness are found at high tide level (Kennedy and Dickson, 2006). These joints are spaced c. 0.5 m apart and are filled with iron oxidation. In areas where joints are open, waves can be very efficient at removing blocks (Trenhaile and Kanyaya, 2007), with wave quarrying occurring along these planes (Bird and Dent, 1966). In Wellington, New Zealand, high densities of open joints (N10/m) can cause lateral erosion rates of up to 0.1 m/year (Kennedy and Beban, 2005). The vertical joints in the Hawkesbury Sandstone in Middle Harbour are generally several metres apart and often filled with iron oxidation. Erosion appears to preferentially occur along these planes, however as the joints are most often spaced many metres apart, and combined with beds over a metre thick, wave quarrying appears limited due to the size of the resulting blocks. This is then reflected in the linear nature of the seaward cliff of the platforms which is defined by the joint sets (Fig. 4b). This finding is similar to the open-ocean coast of Sydney where the structural control was found to be more important than wave intensity (Abrahams and Oak, 1975), allowing platforms to only form where variations in lithology occur (Bird and Dent, 1966). The resistant nature of the Hawkesbury Sandstone as indicated by its compressive strength, massive bedding and widely spaced joint planes, means all the platforms, except Grotto3 at MLWS, have formed at or above MSL. Many in fact rise several metres above the limit of modern marine processes which suggests that erosion may not entirely relate to contemporary processes. Erosion during previous sea level stands often affects the morphology of contemporary platforms (Trenhaile, 2002) especially where the rock resistance may preserve terraces over glacial/ interglacial cycles, such as occurs in Spain (Blancho Chao et al., 2003) and Lord Howe Island, Australia (Dickson, 2006). Along the NSW coast this is an important issue (Stephenson and Thornton, 2005), with the possible inherited nature of platforms in Sydney being debated almost a century ago (Andrews, 1916; Hedley, 1924; Jutson, 1939). In southern NSW elevated platforms have been related to higher sea levels from the previous two interglacials (Bryant et al., 1992; Brooke et al., 1994), and based on surveys of highstand shorelines around Australia these higher sea levels would also have occurred within Middle Harbour (Murray-Wallace and Belperio, 1991; Bryant, 1992). Within the active marine zone in Middle Harbour platform elevations are generally close to MSL although ranging between MLWS and MHWS, with the highest platform within the modern wave zone being found at Washaway Beach at an elevation of +1 m and covered in macroalgae. It is therefore likely that this range would also occur for those platforms developing at a different sea
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level. A distinct horizontal surface is found at Edwards4 over 20 m wide at 1.5 m above MSL and has a relict rampart and possible former water layer weathering pits (Fig. 3a). A similar level occurs at Grotto2 (+1.4 m) and appears to correspond to erosion along a small crossbedded unit (Fig. 3e). A more distinct and spatially continuous surface is found at around +2.2 m. This occurs on the Washaway, Grotto and Edwards Beach sections and may also be represented by a slightly lower surface at + 2 m on Castle Rock1. These 1.5 and 2.2 m surfaces were ascribed to previous levels of marine erosion (Hedley, 1924). A consistent raised platform level of +2 m also occurs along the southern NSW coast and has been dated at the Last Interglacial (LIG) between 80 and 105 ka (Young and Bryant, 1993; Brooke et al., 1994). Young and Bryant (1993) also attribute a higher surface at +4 m to earlier during the LIG at 120–140 ka it is tempting to relate this level to the platforms at + 3.5 m on the Grotto2 and Washaway profiles. It therefore appears most likely that the morphology of the platforms within Middle Harbour is partly inherited from erosive processes during past higher sea levels dating back to the Last Interglacial Period. 6. Conclusions Shore platforms within Middle Harbour, Sydney, form where deepwater ocean waves can impact the rocky shoreline. Joint planes within the sandstone are widely spaced and the massive nature of the beds, combined with their compressive strength of 39 ± 6 means they are relatively resistant to erosion. Near vertical joints appear to characterise the seaward edge of the platforms forming low-tide cliffs. Erosion within the intertidal zones appears to preferentially occur along bedding planes and where these correspond to the intertidal zone, wide platforms are formed. The resistant nature of the lithology means that several platform levels derived from previous higher sea levels occur, namely at c. 1.5–2 and 3.5 m above MSL. Shore platform morphology within Middle Harbour therefore appears to be primarily determined by geological structure, with marine processes only able to exploit existing planes of weakness located within the intertidal zone. Acknowledgements The author would like to thank Michael Kennedy for his assistance in the field. This paper is a contribution to the IAG/AIG working group on Rocky Coasts. References Abrahams, A.D., Oak, H.L., 1975. Shore platform widths between Port Kembla and Durras Lake, New South Wales. Australian Geographical Studies 13, 190–194. Andrews, E.C., 1916. Shoreline studies at Botany Bay. Journal of the Royal Society of New South Wales 50, 165–176. Bird, E.C.F., Dent, O.F., 1966. Shore platforms on the south coast of New South Wales. Australian Geographer 10, 71–80. Blancho Chao, R.B., Costa Casais, M.C., Martinez Cortizas, A.M., Perez Alberti, A.P., Trenhaile, A.S., 2003. Evolution and inheritance of a rock coast: Western Galicia, Northwestern Spain. Earth Surface Processes and Landforms 28, 757–775. Brooke, B.P., Young, R.W., Bryant, E.A., Murray-Wallace, C.V., Price, D.M., 1994. A Pleistocene origin for shore platforms along the northern Illawarra coast, New South Wales. Australian Geographer 25, 178–185. Bryant, E.A., 1992. Late Interglacial and Holocene trends in sea-level maxima around Australia: implications for modern rates. Marine Geology 108, 209–217. Bryant, E.A., Young, R.W., Price, D.M., Short, S.A., 1992. Evidence for Pleistocene and Holocene raised marine deposits, Sandon Point, New South Wales. Australian Journal of Earth Sciences 39, 481–494. Conaghan, P.J., Jones, J.G., 1975. The Hawkesbury Sandstone and the Brahmaputra: a depositional model for continental sheet sandstones. Journal of the Geological Society of Australia 22, 275–283.
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